<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button class="begin">开启定时器</button>
    <button class="stop">停止定时器</button>
    <script>
        var begin = document.querySelector('.begin');
        var stop = document.querySelector('.stop');
        var timer = null; // 全局变量  null是一个空对象
        begin.addEventListener('click', function() {
            timer = setInterval(function() {  // setInterval 设置定时器
                console.log('ni hao ma');

            }, 1000);
        });
        stop.addEventListener('click', function() {
            clearInterval(timer);  // clearInterval 清除定时器
        });
    </script>
</body>
</html>